The invention relates in general to amplifiers, and more specifically to a rail-to-rail gain stage of an amplifier.
Amplifiers are widely utilized in the electronics industry for both amplifying and buffering an input signal and providing an output signal. Of particular interest in industry is to operate the amplifier with a maximum swing between a given set of supplies, and to be able to operate to very low supplies. This function is referred to as rail-to-rail performance in accordance with the operation of the device between the positive and negative voltage supply rails.
A conventional operational amplifier is described in U.S. Pat. No. 5,140,280, which presents a rail-to-rail output stage of an operational amplifier. The output stage of this operational amplifier includes two output transistors, one being a NPN and the other a PNP transistor. The NPN output transistor is driven by an emitter follower configuration, while the PNP output transistor is driven by a current mirror configuration. One observation of such a configuration is that the output drive is not truly symmetrical. In devices which utilize a mirror type of output configuration, there is a limitation on how much current it can source or sink based on the ratio of the transistors in the mirror.
It is therefore an object of the present invention to provide an amplifier gain stage which processes signals operating substantially at the power supply rails.
It is a further object of the present invention to provide a gain stage which is capable of low voltage operation.
It is another object of the present invention to provide a gain stage with a compact design having relatively few transistors in the signal path.
It is yet another object of the present invention to provide a gain stage which has the ability to sink and source large currents while operating on a small bias current.